The present invention relates to a blood volume measuring method and a blood volume measuring apparatus, particular to method, apparatus and program of removing an artifact in an apparatus for analyzing respiratory variation of the stroke volume of the patient in order to determine the fluid response of the patient or adequately set artificial ventilation.
There are related-art methods of analyzing respiratory variation of the stroke volume of the patient. In a first method, the stroke volume is measured from variation of the pulse wave propagation time by using correlation between the pulse wave propagation time and the stroke volume (see JP-A-2005-312947), and respiratory variation is analyzed from the acquired stroke volume.
JP-A-2005-312947 discloses the following biological signal monitoring apparatus shown in FIG. 8. FIG. 8 is a block diagram illustrating the configuration of the related-art biological signal monitoring apparatus, and FIG. 9 is a diagram illustrating an example of the manner of measurement in the related-art biological signal monitoring apparatus. FIG. 10 is a view showing waveforms of pulse waves which are measured by the related-art biological signal monitoring apparatus.
As shown in FIG. 9, a systole/diastole blood pressure measuring unit 120 is configured by a cuff 125, a compressing pump 127, a pressure sensor 128, a cuff pressure detector 129, an A/D converter 122, etc.
Specifically, as shown in FIG. 9, the cuff 125 is attached to an upper arm of the patient for measurement. In the cuff 125, the interior is opened or closed with respect to the atmosphere by an exhaust valve 126 installed in the body 110 of the biological signal monitoring apparatus. Air is supplied to the cuff 125 by the compressing pump 127 installed in the body 110. The pressure sensor 128 is mounted in the body 110, and an output of the pressure sensor 128 is detected by the cuff pressure detector 129. An output of the cuff pressure detector 129 is converted into a digital signal by the A/D converter 122, and input to a cardiac output calculating unit 140. In FIG. 9, the cuff pressure detector 129, the A/D converter 122, and the cardiac output calculating unit 140 are included in the body 110.
In FIG. 10, (a) shows an electrocardiogram waveform, and an aortic pressure wave immediately after the ejection from the heart has a waveform shown by (b). Further, waveforms of an arterial pressure wave at the periphery and a peripheral pulse wave are acquired as shown by (c) and (d).
As shown in FIG. 8, a pulse wave propagation time measuring unit 130 is configured by a time interval detection reference point measuring unit 131, an A/D converter 132, a photoplethysmogram sensor 133, a pulse wave detector 134, an A/D converter 135, etc.
The time interval detection reference point measuring unit 131 is used for detecting a point of time when an R wave is generated on an electrocardiogram, and an output thereof is converted into a digital signal by the A/D converter 132, and then input to the cardiac output calculating unit 140. Specifically, the time interval detection reference point measuring unit 131 is configured by ECG electrodes 131a which are attached to the chest of the patient, as illustrated in FIG. 9. Measurement data is transmitted from a measurement data transmitter 150 which is electrically connected to the ECG electrodes 131a, to the body 110 in a wireless manner. The transmitted measurement data is converted into a digital signal by the A/D converter 132 in the body 110, and then input to the cardiac output calculating unit 140. In this way, the ECG waveform as shown by (a) of FIG. 10 is acquired.
Meanwhile, the photoplethysmogram sensor 133 is intended to be attached to a peripheral part, such as a finger, of the patient, as shown in FIG. 9, and to be used in acquiring the pulse wave propagation time, for example, by performing SpO2 measurement. The photoplethysmogram sensor 133 is electrically connected to the measurement data transmitter 150, and the measurement data transmitter 150 transmits the measurement data to the body 110 in a wireless manner. When the measurement data is sent to the pulse wave detector 134 in the body 110 of the biological signal monitoring apparatus, the pulse wave (photoplethysmogram) at the attachment location of the patient is detected. The output of the pulse wave detector 134 is converted into a digital signal by the A/D converter 135 and then input to the cardiac output calculating unit 140. As such, a waveform of the photoplethysmogram (a waveform at the periphery) such as shown by (d) of FIG. 10 is acquired.
Next, a calculation process of acquiring esCO from an expression of esCO=(αK·PWTT+βK)·HR will be described with reference to the flowchart of FIG. 11. In the expression, esCO is an estimated cardiac output, PWTT is a pulse wave propagation time, HR is the heart rate, and α, β, and K are coefficients inherent to the patient. FIG. 11 shows the procedure in which βK is acquired by calibration using an initial value of αK and then the estimated cardiac output esCO is calculated.
Reading of the initial value of αK is carried out (Step S31).
PWTT and HR are acquired (Step S32).
Next, it is determined whether βK is available or not (Step S33).
If the determination in Step S33 is NO, then a request for input of CO value for calibration is displayed (Step S34).
It is determined whether the CO value for calibration has been input or not (Step S35).
If the determination in Step S35 is YES, the input CO value, and the acquired PWTT and HR are stored in a register as CO1, PWTT1, and HR1, respectively (Step S36).
βK is acquired from an expression of βK=CO1/HR1−αK·PWTT1 (step S37).
Calculation of acquiring esCO from the expression of esCO=(αK·PWTT+βK)·HR is carried out by using the acquired βK (step S38).
If the determination in Step S33 is YES, likewise, calculation of acquiring esCO from esCO=(αK·PWTT+βK)·HR is carried out (Step S38).
The esCO acquired in the calculation is displayed (step S39).
The above process is repeated as required.
From the above esCO, the estimated stroke volume esSV is acquired by using an expression of esSV=esCO/HR.
From the above eeSV, variation of stroke volume (SVV) is acquired by using an expression of SVV=2·(esSVmax−esSVmin)/(esSVmax+esSVmin). In the expression, esSVmax is the maximum estimated stroke volume per respiratory rate and esSVmin is the minimum estimated stroke volume per respiratory rate.
In a second method, the pulse pressure of the blood pressure of the patient is measured by using correlation between the stroke volume and the blood pressure, to measure respiratory variation of the stroke volume (see JP-A-2007-203041). In JP-A-2007-203041, arterial pulsation pressure variation (PPV) is acquired from an expression of PPV=2·(PPmax−PPmin)/(PPmax+PPmin). In the expression, PPmax is the maximum arterial pulsation pressure (PP) per respiratory rate, and PPmin is the minimum arterial pulsation pressure (PP) per respiratory rate.
PPV is calculated with respect to respiratory rates without arrhythmia. When the variability of at least three continuous PPV values (defined as the standard deviation divided by the average value of the arterial pulsation pressure variation PPV) is larger than a predetermined threshold (for example, 15%), the corresponding PPV values are determined to be inadequate, and omitted from dynamic analysis of the blood pressure.
A third method is the PAD (Pulse Amplitude Deviation) method in which, by using correlation between the amplitude of a peripheral pulse wave and the stroke volume, respiratory variation of the stroke volume is measured from variation of the pulse wave amplitude (see JP-A-2004-105682). In JP-A-2004-105682, the pulse wave amplitude variation (PAV) is acquired by an expression of PAV=100·(Pmax−Pmin)/meanP. In the expression, Pmax is maximum pulse wave amplitude per respiratory rate, Pmin is minimum pulse wave amplitude per respiratory rate, and meanP is average pulse wave amplitude per respiratory rate.
In the first method where respiratory variation of the stroke volume is measured from variation of the pulse wave propagation time by using correlation between the pulse wave propagation time and the stroke volume, there is a difference between the measurement resolutions of an ECG and a pulse oximetry plethysmogram and the measurement resolution of the pulse wave propagation time. In the case where the measurement resolution of an ECG is 4 msec and that of a pulse oximetry plethysmogram is 8 msec, for example, the measurement resolution of the pulse wave propagation time is 3 to −7 msec, and insufficient. Therefore, the process requires an A/D converter which has a higher measurement resolution of, for example, about 1 msec. Since an ECG and a pulse oximetry plethysmogram are used, there is a problem in that the method is sensitive to an artifact.
In the second method, the minimum resolution of the blood pressure which is usually used in clinic is about 1 mmHg, and requested resolution is not so high. In the second method where, by using correlation between the stroke volume and the blood pressure, the pulse pressure of the blood pressure of the patient is measured to measure respiratory variation of the stroke volume, by contrast, the blood pressure is directly measured by using an arterial catheter, and hence there is a problem in that the measurement of the blood pressure waveform may be often interrupted by zero balance, flash, or the like, and the interrupted portion must be omitted from the analysis.
In the third method which is the PAD (Pulse Amplitude Deviation) method where, by using correlation between the amplitude of a peripheral pulse wave and the stroke volume, respiratory variation of the stroke volume is measured from variation of the pulse wave amplitude, the pulse wave amplitude is largely varied depending on the peripheral circulation and vasomotor innervation, and changed in the range of 100 or more times. In order to cope with a high-amplitude pulse wave while ensuring the resolution required for a low-amplitude pulse wave, therefore, an A/D converter having a wide dynamic range and a high resolution is necessary. In a pulse oximetry plethysmogram, the signal level is low, and the measurement is performed on the surface of the body. Therefore, there is a problem in that the method is sensitive to an artifact.